Today several different types of ear simulators are on the market. These ear simulators are typically used in different situations where it is required to simulate an input/transfer impedance of human ears. A simple so-called 2 cc coupler is used for measuring and verifying acoustic performance parameters of portable electroacoustic or communication equipment such as hearing instruments, head-sets, handsets, ear insert phones etc. during manufacturing. The 2 cc coupler comprises a volume of 2 cm3 of simple geometrical shape providing a rough representation of the input impedance of an average human ear canal.
Several manufacturers offer more advanced types of ear simulators in form of the so-called 711 couplers complying with IEC 60318-4 and ANSI S3.25 standards. One type of 711 coupler is manufactured and marketed by Brüel & Kær Sound and Vibration Measurement A/S under the designation “Ear Simulator—Type 4157”.
The 711 type of couplers are constructed to closely reproduce acoustic parameters of average human ear canals by presenting an appropriate input impedance or transfer impedance at a reference measurement plane at which a sound reproducer to be tested is placed. The sound reproducer may comprise an acoustic transducer such as an electro-dynamic, piezoelectric, moving armature loudspeaker of the piece of portable electroacoustic equipment to be tested. The fact that 711 couplers are constructed to closely reproduce acoustic parameters of average human ear canals means that the input impedance or transfer impedance of a 711 coupler is designed to be representative of, or model, a combination of the input/transfer impedance of average human ear drums and the input/transfer impedance of an average human ear canal. These two factors are thus inseparable in any acoustic measurement based on a 711 type ear simulator.
The 4157 ear simulator comprises a main housing assembled of a number of discs whose shapes form annular air volumes coupled to a main volume of the housing by air passages or channels. A ½ inch measurement microphone is mounted at a measurement plane of the main volume to model the position of an eardrum in real human ears. The reference plane of the 4157 ear simulator is located at an entrance of the main volume and as previously mentioned, the position at which the sound outlet of the sound reproducer is placed to ensure the input/transfer impedance of the 4157 ear simulator is accurate. Furthermore, the transversal cross-sectional profile of the main volume of the 711 type ear simulator is oriented substantially parallel to the measurement plane (wherein the diaphragm of the measurement microphone is situated). This orientation of the measurement plane does not accurately mimic the orientation of a human ear drum which is tilted relative to the human ear canal at the intersection between the ear drum and the ear canal.
However, the input impedance or transfer impedance of the 4157 ear simulator, and other 711 type ear simulators, is only considered to be accurate at frequencies up till about 7 kHz. Above this frequency, it has not been possible to verify how well the input or transfer impedance represents the average input or transfer impedance of real human ear canals. The input impedance or transfer impedance of 711 type ear simulators rises abruptly about 13.5 kHz due to a half-wave resonance in a longitudinal dimension of the main volume. This fact is also expressly acknowledged in the IEC 60318-4 and ANSI 83.25 standards which only call for accurate impedance representation till about 8 kHz for compliant ear simulators.
Furthermore, in the frequency range below 8 kHz it has not so far been possible to verify how accurately the acoustic impedance at the measurement plane (or microphone diaphragm position) of the 711 type ear simulator represents the average impedance of human ears at the ear drum, i.e. ear drum impedance. This lack of verification is due to the assumption made when transforming a measured input impedance of human ear into corresponding ear drum impedance. Large errors are easily introduced in this transformation due to a lacking of knowledge of the geometry of the ear canal volume enclosed between the measurement probe and the human ear drum.
The lack of accuracy and unknown performance of the 711 type ear simulators is undesirable in view of a continuing trend of reproducing sound with increased fidelity and frequency extension such as to frequencies above 10 kHz, or even above 12 kHz, in today's portable communication equipment. It would accordingly be highly desirable to provide an ear simulator which accurately represents average input impedance or transfer impedance of real human ear canals so as to allow this type of broad band or high frequency capable portable communication equipment to be properly evaluated.
Furthermore, it would also be highly advantageous to provide an ear simulator assembly that was capable of taking differences in ear canal geometry between different human populations such as infants, children, Asian males etc. into consideration. Such an ear simulator has been provided by the present inventors by firstly measuring and computing the ear canal input impedance of a representative human population throughout an extended frequency range both below and above 16 kHz. Secondly, the present inventors have successfully transformed these broad-band ear canal impedance measurements to corresponding ear drum impedances. Thirdly, by designing an ear simulator (“ear drum simulator”) representing the average ear drum impedance of the human population. The present ear drum simulator is highly useful in numerous applications requiring accurate acoustic modeling of human ear canals. For example, the ear drum simulator may be coupled to a detachable or user selectable separate ear canal simulator, representing a known input/transfer impedance of average human ear canals of a target population, or representing a known input/transfer impedance of a particular individual, to provide a flexible ear simulator assembly. This feature makes it possible to construct or assemble a customized ear simulator assembly accurately representing the average ear canal input impedance of the target population or accurately representing the ear canal input impedance of a specific individual.
The customized ear simulator assembly enables accurate prediction of sound amplification and ear drum sound pressure on the specific individual delivered by a piece of portable communication equipment. This property is of considerable advantage in a plurality of applications such as hearing instrument fitting procedures where known ear simulators are solely capable of estimating average sound amplifications and average ear drum sound pressures. These averages may depart considerably from real values on a specific individual or patient due to intra-subject variations in ear canal geometries and input impedances. In hearing instrument fitting procedures, this lack of accuracy is highly undesirable because the hearing impaired user or patient may receive too small or to large sound amplification to adequately compensate for his/hers hearing loss or may be exposed to uncomfortably loud maximum sound pressure levels.